A plurality of wireless communications services are now available for communicating between a central site and multiple mobile or portable users. A trunked radio system, one such service, allows portable or mobile users (for example, public safety or emergency service providers) to communicate with others in an assigned radio group and a group dispatcher, over assigned frequency channels within a defined geographic region. When the user desires to talk to the dispatcher or another user in the radio group, he activates a push-to-talk “button” on his wireless communications device. The device (sometimes referred to as a radio in the public safety system) transmits an inbound control signal received at a base station. Responsive thereto, the base station assigns a frequency channel for the user's call and transmits an outbound control signal that forwards the frequency assignment to all communications device within the radio group. The user can then send a voice or data message on the assigned channel; the message will be heard by all users in the radio group.
A cellular telephone system comprises a plurality of cells, each having a transmitting/receiving base station for sending and receiving signals from the mobile users operating within the cell according to known system protocols (e.g., CDMA, AMPS, TDMA, etc.). As the cell phone user moves into the coverage area of an adjacent cell, the cell phone signal is handed off from the base station of the original cell to the base station of the adjacent cell.
In a wireless communications systems, such as the cellular telephone or trunked radio systems, it is sometimes desired to determine a mobile user's location. For example, to provide emergency services to a mobile cellular user, the cellular system includes an emergency services feature, such as a “911” service (referred to as E911 or enhanced 911 service). When a mobile user requiring emergency services initiates a 911 call a geolocating process determines the calling party's location so that emergency personnel can be dispatched to the site. A public safety, dispatcher operating over the trunked radio system may need know each user's location within the service region to properly deploy personnel to a specific site as required to render needed services. Knowing the location allows the dispatcher to efficiently dispatch service providers to locations where the services are required.
It is known that a GPS (global positioning satellites) system is commonly used to determine a location, such as the location of a mobile or portable wireless communications system user. A GPS receiver receives and processes time-based signals from at least three GPS satellites to determine its, and the users, location. However in certain situations it is also desired for another party associated with the communications system, such as a public safety dispatcher or emergency services provider, to know the location of the mobile or wireless user. After the GPS receiver determines the user's location, a communications device must convey the location information to the other party, at the expense of time and communications bandwidth. Further, location accuracy of a GPS system suffers when interference disrupts the signal from one or more of the three satellites, such as when the receiver is inside a structure or when the line of sight from the receiver to the satellites is obscured.
In certain applications it is therefore desired to determine the users location by other more robust techniques capable of accurate location determination without the necessity of receiving three satellite signals, thereby avoiding the detrimental aspects of the GPS system. It may also be desired for the determined location to be supplied to another party, such as the dispatcher or services provider mentioned above.
Time difference of arrival (TDOA) (also referred to as geometric triangulation) geo-location is one known technique for accomplishing these objectives. A signal transmitted from the mobile or portable user's device is received by at least three receiving sites (typically ground-based receiving sites). The pairwise time difference of arrival at all receiving site pairs is determined by correlating the signal received at one site of the pair with the same signal received at the other site of the pair. To ensure that each site of the pair processes the same received signal, a known and unique portion of the signal (a preamble with a known format, for example) is commonly detected and used in the correlation process.
The relationship between distance and time is given by d=ct, where c is the speed of light, t is the transit time of the signal and d is the distance between a transmitting and a receiving site. Thus the time difference between receipt of a signal at two receiving sites corresponds to the difference in distance between the transmitting site and each of the two receiving sites. With the TDOA approach, a signal received at n receiving sites yields n(n−1)/2 pairs of time difference of arrival values that determine the location of the wireless communications device. Each pairwise TDOA value generates a locus of points in the form of a hyperbola. An intersection of at least three hyperbolas uniquely determines the location of the communications device.
The signal received at each receiving site is nearly identical, with differences typically caused by channel impairments, non-identical noise characteristics encountered in the signal paths between the wireless communications device and the receiving sites, multipath effects, fading, non-identical analog processing components at the receiving sites. The detection accuracy of the receiver at the receiving sites directly affects the TDOA value and therefore directly affects location determination accuracy. A detection process at each site that minimizes a bit error rate and false alarm rate produces a more accurate location determination.
As illustrated in FIG. 1, according to the prior art time-difference-of-arrival technique, a wireless communications device 2 transmits a radio frequency (RF) signal modulated by a baseband information signal (message) that is received at plural receiving stations 4. The RF signal may comprise, for example, a control signal (i.e., for use in controlling operation of the device 2). In the example system of FIG. 1, the plural receiving stations 4, designated RX1, RX2 and RX3, have a common time reference (not shown), such as a signal received from a high precision clock on a geosynchronous satellite, a signal received from a central processing site or highly stable internal clocks (which can be periodically updated or synchronized to an external time reference).
The signal received at each of the receiving stations 4 is detected and the baseband information signal supplied to the central processing site 6 where the signals are pairwise correlated to determine the pairwise TDOA information. A system employing this technique is referred to as a centralized cross-correlation system. The time difference of arrival is calculated in the time domain by deterring the cross-correlation function for the two baseband information signals. The correlation is carried out directly according to the cross-correlation integral or by the discrete time equivalent of the cross-correlation integral, to generate the cross-correlation function. The cross-correlation is a function of the offset between the two signals, and the cross-correlation function peak (the point of highest correlation between the two signals) identifies the time offset between the signal received at the two receiving sites. That is, the time difference of arrival of the signal at the two receiving base stations equals the time offset at the function peak.
The cross-correlation function peak is typically determined by sampling the cross-correlation function and comparing samples until the peak is found. In this search process there can be some difficulty in accurately determining the peak's location because the peak is unlikely to fall directly on a time sample. Thus, if time difference of arrival values are to be determined more precisely than the sampling period, the cross-correlation function must be interpolated between sample periods to find the function peak. Alternatively, the cross-correlation function can be over-sampled during die peak search process, but this is accomplished at the expense of increased computational overhead and processing time.
As described above, channel impairments, noise, multipath effects, non-identical processing in the receiving site signal paths, etc. affect signal detection accuracy at each site, in turn affecting the accuracy of the cross-correlation results and the peak location. The accuracy of the determined user's position also depends on the sharpness/narrowness of the correlation peak. Determination of the correlation peak is improved by limiting bit errors and false alarms in the detected signal. Thus improving die integrity of the signal detection process consequently improves the accuracy of the determined location.
Generally, the time difference of arrival of the same signal at any two receiving stations is a constant (ignoring motion of the wireless communications device) and assuming flat terrain, yields a locus of points along a hyperbola. (If the terrain is not flat, a hyperbola of rotation is the surface to be considered as it intersects the terrain.) For example, with reference to the schematic representation of FIG. 2, the possible locations of a wireless communications device transmitting a signal that arrived at receiving site RX1 at t1 and arrived at receiving site RX2 at t2 are defined by a locus of points forming a hyperbola curve 16, where the curve is defined by t2−t1=k1, where k1 is a constant. Clearly it is not possible to determine a precise location of the wireless communications device 2 with only two receiving sites reporting time-of-arrival information. Including the signal TOA information from the receiving site RX3 generates two additional curves based on the signal's time difference of arrival at the three receiving sites taken in pairs. One such curve is determined by the time difference of arrival between receiving stations RX1 and RX3 (curve 17), and the other is determined by the time difference of arrival between receiving stations RX2 and 1R3 (curve 18). The intersection of the curves 16, 17 and 18 is the geolocation of the wireless communications device 2.
It is known that in certain situations it may be possible to determine the location of the wireless communications device using the TDOA results from three receiving stations bar calculating only two time difference of arrival values. The two generated hyperbolae will intersect at two points and thus the mobile location is not uniquely determinable. However, when the two points of intersection are overlaid with an area map showing various man-made and natural features, it may be possible to eliminate one of the two intersections as a possible mobile location. For example, if it is assumed that the wireless communications device is on a road, and only one of the two curve intersections occurs along a road, then the other potential location is eliminated from consideration. Alternatively, one of the receiving sites may be able to determine the sector of the azimuth plane (or the angle of arrival) from which the wireless communications device signal is received. When coupled with the two possible locations at the intersection of the two hyperbolae, this angle of arrival information permits selection of the actual location from the two possible locations.